The present invention relates to an oil-free screw type vacuum pump, provided with a suction port and a discharge port, for discharging gas. The vacuum pump according to the present invention is preferably applicable to discharge such gas that is sucked from a process chamber during surface treatment in a process for manufacturing, for example, semiconductor products, electronic products and so on, and that contains gaseous components which may risk operation of the vacuum pump.
In the manufacturing process of the semiconductor products, there have been employed a chemical vapor deposition apparatus (CVD) for forming, on a surface of an object to be treated, a film such as a poly silicon film, a silicone nitride film, a silicone oxide film, etc., an etching apparatus for conducting surface treatment by flowing reaction gas, and so on.
The chemical vapor deposition apparatus (CVD), for example, includes a reaction chamber, a heater for heating this reaction chamber, a furnace port flange provided below this reaction chamber, a stock gas supply system connected to the furnace port flange, and an exhaust system connected to an upper part inside the reaction chamber. A silica boat carrying thereon a number of objects to be treated is arranged at a predetermined position in the reaction chamber, and the stock gas such as SiH4, SiH2Cl2, NH3, etc. is supplied to the reaction chamber under a reduced pressure and while heating, to form a reaction-formed film such as a poly Si film, a silicone nitride film, etc. on the objects to be treated.
In the above described chemical vapor deposition apparatus (CVD) and the etching apparatus, the gas required for producing the semiconductors has been supplied to a wafer which is arranged inside a vacuum chamber to conduct the treatment of the wafer. All the gas after the treatment in the vacuum chamber has been sucked from the vacuum chamber by means of a vacuum pump, and thereafter, has been exhausted to the atmosphere or to a treatment device through an outlet of the pump.
In the manufacturing process of the semiconductors, an oil-free dry vacuum pump has been employed so that oil molecules may not leak to a process chamber. Conventionally, as the vacuum pump for the apparatus for manufacturing the semiconductors, besides using singly the dry vacuum pump such as a screw type pump, a turbo molecule pump, etc., there has been employed a multi-step vacuum pump including a plurality of the pumps combined together to increase exhausting efficiency. As shown in FIG. 3, the screw type dry vacuum pump has two shafts 101, 102 held in parallel in a casing 100. Screw rotors 103, 104, which have screw grooves in mesh with each other, are fixed to the shafts 101, 102. One of the shafts, i.e. the shaft 101 is driven to rotate by means of a motor 105, and the rotation is transmitted to the other shaft 102 by way of a gear 106 which is provided at the end of the shaft 101. The casing 100 is provided with a suction port 107 and an exhaust port 108. With the above-described structure, by synchronously rotating the screw rotors 103, 104 in reverse directions, the gas enclosed between the screw rotors 103, 104 and the casing 100 is moved by the rotation in an axial direction and exhausted.
Function of the screw type vacuum pump includes a suction step of sucking the gas from the suction port 107 into the rotors, a transfer step of transferring the gas within the rotors, and a discharge step of discharging the gas from the exhaust port 108.
During these steps, formation of solid matters in the pump becomes a problem. Precipitation and formation of the solid matters may be carried out through chemical reactions between gas components to be exhausted by the pump, reactions of the gas components on a surface, and/or catalytic effects. Besides such chemical reactions, formation of the solid matters may occur also with a change of agglomeration state caused by a rise of pressure or cooling.
Moreover, the exhaust port 108 of the conventional screw type dry vacuum pump has been provided in a bottom part of the housing, as shown in FIG. 4, in order to increase exhausting efficiency. Therefore, because bearings 109 for rotatably fixing the screw rotors have been arranged in the bottom part, a sectional shape of the exhaust port has been restricted in size.
The suction port 107 has been provided in a ceiling portion of the casing 100 in order to increase a conductance because the vacuum pump, generally arranged vertically, is disposed just below the vacuum chamber.
In recent years, there has been a tendency for the apparatus for manufacturing the semiconductor to use a large amount of gas, because the wafer has become large-sized in diameter and a film of high quality has been aimed. Only a portion of the gas which has been introduced into the vacuum chamber in the apparatus for manufacturing the semiconductor has contributed to the reaction, and most of the remaining gas has been exhausted without reacting. All the remainder of the reaction-forming gas has passed through the vacuum pump, and the reaction-forming gas to be exhausted from the vacuum pump has been liable to coagulate in the exhaust port due to a rise of pressure. Especially, in the vacuum pump such as the screw type dry vacuum pump which has a function of exhausting by pressurizing the gas, the pressure of the reaction-forming gas has risen in the pressurizing process just before exhaustion, and solid products have been created because of a change of the coagulation state near the exhaust port. Conventionally, the exhaust port of the screw type dry vacuum pump has been generally provided in a bottom face at an exhausting side of the housing which contains the screws, and the gas has been exhausted from the bottom face. However, in case of exhausting the gas from the bottom face, it has been impossible to make a sectional area of the exhaust port large, because there has been provided a structure including the bearings for fixing the screw rotors and seal mechanisms for preventing a leak of lubricating oil to the exhaust chamber. For this reason, there has been such a problem that when the above described solid products have been adhered to the exhaust port and deposited there, the solid products may block the exhaust port in a short period, and possible working time of the vacuum pump may be decreased.
Further, with respect to the conventional exhaust port, it has been necessary to completely isolate the exhaust port from a lubricating chamber which has been filled with oil, and to form an exhaust passage for introducing the exhaust gas to the exterior of the vacuum pump. For this reason, the produced coagulum has adhered to an inner face of the exhaust passage, while the reaction-forming gas has flowed through the passage, and the sectional area of the exhaust port has been decreased, resulting in deterioration of exhausting ability. There has also been such a problem that because the exhaust passage having the narrow sectional area runs through the complicated structure including the bearings, the seal mechanisms for preventing a leak of the lubricating oil to the exhaust chamber, and so on, time for maintenance for removing these adhered deposits has become inevitably long.
The adhesion of the produced coagulum has occur also on the portion of the housing in the vicinity of the suction port.